The present invention generally relates to fabrication processes that include a joining operation. More particularly, this invention is directed to a technique for fabricating rotating hardware, as an example, rotating components of a turbomachine, joining techniques used in their fabrication, and the hardware formed thereby.
Components within the combustor and turbine sections of a gas turbine engine are often formed of superalloy materials, typically titanium-, cobalt-, nickel-, and steel-based alloys, in order to achieve acceptable mechanical properties while at elevated temperatures resulting from the hot combustion gases produced in the combustor. Higher compressor exit temperatures in modern high pressure ratio gas turbine engines can also necessitate the use of high performance superalloys for compressor components, including blades, spools, disks (wheels), blisks, and other components. Suitable alloy compositions and microstructures for a given component are dependent on the particular temperatures, stresses, and other conditions to which the component is subjected. For example, airfoils such as compressor and turbine blades (buckets) and vanes (nozzles) are often formed as equiaxed, directionally solidified (DS), or single crystal (SX) superalloy castings to withstand the high temperatures and stresses to which they are subjected within their respective sections of a gas turbine engine, whereas the rotating hardware to which the airfoils are mounted, such as turbine disks and compressor spools and disks, are typically formed of superalloys that must undergo carefully controlled forging, heat treatments, and surface treatments to produce a controlled grain structure and desirable mechanical properties. Because of their different requirements, airfoils and their supporting rotating hardware are typically formed from different alloys. Particular examples of blade alloys include steels such as A286 and AM-355, titanium-based alloys such as Ti-6Al-4V and Ti-8Al-1V-1Mo, cast and wrought polycrystalline gamma prime (γ′) precipitation-strengthened nickel-based alloys such as U720, IN718, and cast mono-crystal or single-crystal gamma prime precipitation-strengthened nickel-based alloys such as MX4 (U.S. Pat. No. 5,482,789), René N5 (U.S. Pat. No. 6,074,602), RenéN6 (U.S. Pat. No. 5,455,120), CMSX-10, CMSX-12, and TMS-75. Particular examples of disk alloys include gamma prime precipitation-strengthened nickel-based alloys such as René88DT (U.S. Pat. No. 4,957,567), René104 (U.S. Pat. No. 6,521,175), and certain nickel-base alloys commercially available under the trademarks Inconel®, Nimonic®, and Udimet®.
In view of the above, blisks (also referred to as bladed disks and integrally bladed rotors) used in compressors of gas turbine engines have often necessitated certain compromises because their disks and blades are manufactured as a single integral part, as opposed to manufacturing the disks and blades separately and then mechanically fastening the blades to the disk. FIG. 1 is a fragmentary cross-sectional representation of a blisk 10 of a type that may be used in a gas turbine engine. The blisk 10 is represented as having a unitary construction that includes a rim 12, disks 14 (wheels), and blades (buckets) 16 (of which only one is shown). Each disk 14 has a bore (hub) 18 at its radially innermost extent and a relatively thinner web 20 between its bore 18 and the rim 12. A through-hole 22 is centrally located in the bore 18 for mounting the disk 10 on a shaft (not shown) driven by the turbine section of the engine, and therefore the common central axis 24 of the disks 14 coincides with the axis of rotation of the blisk 10. Other aspects of the blisk 10 and the construction and operation of the compressor and engine in which the blisk 10 may be installed are known in the art and therefore will not be discussed here in any detail.
The weight and cost of single-alloy blisks of the type represented in FIG. 1 have driven the desire to develop materials, fabrication processes, and hardware designs capable of reducing forging weight and costs. One approach is prompted by the fact that the bores and webs of blisks have lower operating temperatures than their rims and blades, and therefore can be formed of alloys with properties different from those required by the rims and blades. Depending on the particular alloy or alloys used, optimal microstructures for the bore, web, rim and blades also typically differ. For example, a relatively coarse grain size may be optimal for the rim to improve tensile strength and resistance to low cycle fatigue, while a finer grain size may be optimal in the bore and web for improving creep, stress-rupture, and crack growth resistance.
In U.S. Published Patent Application Nos. 2008/0120842 and 2008/0124210, multi-alloy rotor assemblies are described as fabricated by separately forming the bore and rim of a disk from different materials and then joining the bore and rim in the web region therebetween using a metallurgical joining process. A variety of joining techniques are available for this purpose, such as inertia welding as disclosed in U.S. Published Patent Application No. 2008/0124210. As rotor assemblies, blades are retained in slots at the perimeter of the rim, instead of being manufactured as a single integral part with the rim as in the case with blisks. Accordingly, blisks pose an additional challenge to the fabrication of multi-alloy rotating hardware using the teachings of U.S. Published Patent Application Nos. 2008/0120842 and 2008/0124210.